1. Field of the Invention
This invention is directed to extruded plastic filaments, and in particular to a filament used as a cutting medium, in applications such as grass trimmers; and to apparatus and process for the manufacture thereof.
2. Description of the Prior Art
The use of extruded plastic filament as a cutting agent for grass trimmers is widespread.
In most instances the filament is mounted on a reel that is supported on an inverted rotatable spindle of a vegetation trimming machine. Such machines have a safety guard that at least partially overlies the extended filament, such that in operation the filament can contact vegetation such as grass, light shrubs, weeds and the like in cutting reaction therewith, due to the speed of rotation of the machine.
As a consequence, the free tip of the filament, which does much of the cutting is progressively abraded, requiring the release of a compensatory length of the filament from its reel in order to continue operation. Existing filaments are very susceptible to damage when coming into contact with common obstacles, particularly masonry, stones, drainpipes, etc.
The plastic resin presently most used consists of a nylon or a nylon/polyethylene copolymer, extruded through a die.
Such prior art filaments are subject to a high rate of breakage at the tip when impacting hard objects such as posts, cement blocks, walls, down-spouts and the like. Ribbed filament sections of the prior art, being of smooth finish, do not provide a very effective cutting profile.
In manufacturing such filaments, difficulties have been experienced in maintaining a true profile, due to the flattening effects of gravity on horizontally oriented extrudate. With higher temperature extrusions, the problems of diametical distortion have increased, so that the maintenance of tolerances is most difficult. Certain prior art trimmer filaments possess an inadequate High Heat Resistance (H.H.R.) value, so that in the high temperature environment of sustained operation self adhesion takes place.
The present invention provides an improved cutting filament for use in a rotary trimmer.
In one embodiment, a polypropylene filament is extruded, and subjected to an orientation process, to enhance the strength and toughness of the material.
In another embodiment, a polypropylene or polypropylene blended with polyethylene of low or of high density, extruded as an oversize polygonal section, is drawn down in size, being found to possess improved abrasive, herbiage-cutting characteristics.
A further embodiment has a finned profile with at least one fin in the form of a projecting rib. In the case of a polygonal section, this is preferably modified, wherein fin portions of the section are located at the apices of the polygon.
The application of a drawdown molecular orientation process enables the use of an extrusion die of sufficient size that the finning provision is readily incorporated into the die. The effective cutting action of the fin or fins is greatly enhanced when the sides and bottom surfaces of the die grooves are scarified, to be irregularly rough and jagged.
The present invention thus provides an extruded filament for use as an abrasion medium, wherein the filament consists of a polyolefin mixture having the molecular structure thereof at least partially axially oriented, to impart enhanced strength and abrasion resistance to the filament.
In particular, the filament is found to be useful as a cutting medium in a rotary herbiage trimming tool.
It has been found in one embodiment that a mix of plastic resins consisting of 20 to 40 percent by weight of linear low density polyethylene is mixed with 60-80 weight percent polypropylene copolymer or homopolymer as the balance a high strength, impact and abrasion resistant, pliable filament of adequate High Heat Resistance (HHR) is produced, which is able to withstand use as a trimmer medium without self-welding, in the usual hot working environment.
As an example, a Montell polypropylene, listed as an extrusion grade homopolymer No.6823, and with characteristics of MFRxe2x80x940.5 dg/min; Tensile Strengthxe2x80x944800 psi; Flexural modulusxe2x80x94180,000 psi; IZOD Impactxe2x80x941.5 ft-lb/in; HDTxe2x80x94205 F. degrees; rated Underwriters Lab 94HB, and with FDA Y1 rating gave excellent working results when extruded and processed in accordance with the presently disclosed process, with a service life improvement as a trimmer medium of more than 40 percent over standard commercial nylon trimmer filament.
In another example, Montell""s SV-152 (nucleated) super high impact copolymer is used.
Some of these copolymers have a low Melt Flow Rate (MFR) of 2 or less, requiring extrusion at 500 F. degrees and higher; the lower the MFR value, the higher is the required extrusion temperature.
The combining of polyethylene resin, of high or low density to the polypropylene improves pliability.
The polypropylene supplied may incorporate high density polyethylene as one of its constituents.
One embodiment of the present filament has a polygonal cross section, wherein the apices of the polygon appear to serve as cutting edges, when employed with a rotary tool as an abrading agent.
A further embodiment of the filament may have a finned profile, to enhance the abrading action of the filament.
In the case of a filament having a polygonal profile, it may have at least one fin portion extending from an apex of the polygonal profile.
The finned filament may have a number of the fins, as in the case of a polygonal section, having each fin extending individually from a respective apex of the polygonal profile.
The provision of a scarified finish to the working surface of a filament has been found to greatly enhance the abrading effectiveness of the filament. This applies to plain, ribbed and circular section filaments.
In view of the drawing-down of the extruded section, a die of sufficient size is used wherein a scarified finish may be applied to surface portions of the die, using a toothed engraving tool.
The process of providing an oriented filament in accordance with the present invention includes the steps of providing a die of predetermined size, form and finish; extruding a selected plastic resin at a first, extrusion temperature in a predetermined first temperature range; minimally cooling the extrudate to a first handling temperature; passing the extrudate through a first drawing apparatus operating at a first linear speed; heating the extrudate to a second, orienting temperature in a predetermined second temperature range; drawing-down to a filament the re-heated extrudate at a second linear speed in the range six to nine times the first linear speed, to orient the structure of the filament in a substantially axial direction by tensioning by means of a high speed orienting puller, including the step of cooling the oriented filament prior to passage thereof through the orienting puller; and coiling the oriented filament on a spool winder. In the case of a resin having a high percentage content of polypropylene, a normal cooling bath may be somewhat inadequate, due to the high extrusion temperature required for this material. It has been found that a venturi cooler may be employed, having a conical shape, through which the extrudate is passed.
Coolant admitted at the throat flows about the extrudate in effective cooling relation as it passes through the diverging shape of the conical cooler, usually into a cooling bath.
In the case of a circular section filament, in order to maintain a substantially circular extruded section, within reasonably specified tolerances, and to avoid the section flattening that can readily occur with high temperature extrusion, the extrusion die is preferably oriented with its polar axis vertical, discharging substantially directly into a cooling bath.
In an extrusion head having an annular array of die orifices, a dispersaL cone is located upstream of the die orifices, to facilitate uniformity of product mix.